Periodic permanent magnet electron beam focusing arrangement for traveling-wave tubes having plural interaction cavities in bore of each annular magnet



J1me 1967 L. M. WINSLOW ETAL 3,324,339

PERIODIC PERMANENT MAGNET ELECTRON BEAM FOCUSING ARRANGEMENT FOR TRAVELING-WAVE TUBES HAVING PLURAL INTERACTION CAVITIES IN BORE OF EACH ANNULAR MAGNET Filed Feb. 27, 1964 3 Sheets-Sheet 1 Armin 5y June 6. 1967 M. WINSLOW ETAL 3,324,339 PERIODIC PERMANENT MAGNET ELECTRON BEAM FOCUSING ARRANGEMENT FOR TRAVELING-WAVE TUBES HAVING PLURAL INTERACTION CAVITIES IN BORE OF EACH ANNULAR MAGNET Filed Feb. 27, 1964 3 Sheets-Sheet 2 126 32a /2 I L 1 A/ .s 5 4/ 34/ /V 34 4 Z 38 a 3 v 0 J 4 6 i2 55 g 52+ I 4i .1? A a? I i V W3; .J

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J1me 1967 L. M. WINSLOW ETAL 3,324,339

PERIODIC PERMANENT MAGNET ELECTRON BEAM FOCUSING ARRANGEMENT FOR TRAVELING-WAVE TUBES HAVING PLURAL INTERACTION CAVITIES IN BORE OF EACH ANNULAR MAGNET Anne/5V.

United States Patent C 3,324,339 PERIODIC PERMANENT MAGNET ELEC- TRON BEAM FGCUSING ARRANGEMENT FOR TRAVELING-WAVE TUBES HAVING PLURAL INTERACTION CAVITIES 1N BURE OF EACH ANNULAR MAGNET Lester M. Winslow, Los Angeles, and David J. Bates, Los Altos, Calitl, assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Feb. 27, 1964, Ser. No. 347,771 7 Claims. (Cl. 315-35) This invention relates generally to traveling-Wave tubes, and more particularly relates to a periodic permanent magnet focusing arrangement for a coupled cavity traveling-wave tube which produces a periodically varying axial magnetic focusing field having at least one additional variation in magnetic flux density every magnetic half period.

In traveling wave tubes a stream of electrons is caused to interact with a propagating electromagnetic Wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure, such as a conductive helix wound about the path of the electron stream or a folded waveguide type of structure in which a waveguide is etiectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence, the traveling-wave may be made to effectively propagate at nearly the velocity of the electron stream. Interactions between the electrons in th stream and the traveling-wave cause velocity modulations and bunching of the electrons in the stream. The net result may then be a transfer of energy from the electron stream to the wave traveling along the slow-Wave structure.

The present invention is concerned with traveling-wave tubes utilizing slow-wave structures of the coupled cavity, or interconnected cell, type. In this type of slow-wave structure a series of interaction cells, or cavities, are disposed adjacent to each other sequentially along the axis of the tube. The electron stream passes axially through each interaction cavity, and electromagnetic coupling is provided between each cavity and the electron stream. Each interaction cavity is also coupled to an adjacent cavity by means of a coupling hole in the end wall dcfining the cavity. Generally, the coupling holes between adjacent cavities are alternately disposed on opposite sides of the axis of the tube, although various other arrangements for staggering the coupling holes are possible and have been employed. When the coupling holes are so arranged, a folded waveguide type of energy propagation results, with the traveling-wave energy traversing the length of the tube by entering each interaction cavity from one side, crossing the electron stream and then leaving the cavity from the other side, thus traveling a sinuous, or serpentine, extended path.

Since the electron stream is projected along the axis of the tube through minimum sized holes in the end Walls of the interaction cavities, or more generally as proximate to the slow-wave circuit as possible, the electron stream must be precisely constrained to its axial path in order to prevent excessive impingement of electrons on the slow-wave circuit. Generally, this is acccomplished by immersing the electron stream in a strong axial magnetic field which tends to provide the required focusing so that the electron stream may pass as closely as possible to the slow-wave structure without excessive interception of electrons by the slow-wave structure. In one of the early techniques for providing the constraining axial magnetic field, the slow-Wave structure is aligned concentrically within a long solenoid wound of a conductor carrying a relatively large electrical current. Another early focusing scheme for traveling-wave tubes involves the use of a single large permanent magnet, of a length substantially equal to that of the slow-Wave structure, disposed about the slow-wave structure with a pole piece at each end of the magnet. While solenoids and permanent magnets have been able to provide satisfactory focusing, the excessive size and Weight of these focusing arrangements have made tubes focused in this manner impractical for many mobile applications.

In order to provide more compact focusing means for traveling-Wave tubes, periodic permanent magnetic focusing arrangements were developed in which a plurality of short annular permanent magnets are disposed in axial alignment along and about the slow-wave structure with a plurality of annular ferromagnetic pole pieces interposed between and abutting adjacent magnets. The magnets are magnetized axially and arranged with like poles of adjacent magnets confronting one another so that there is produced, along the axis of the tube, a periodic magnetic field of sinusoidal distribution, with zero field occurring at each pole piece and with a period equal to twice the pole piece spacing.

A significant step in the advancement of the travelingwave tube electron beam focusing art was made possible by the development of coupled cavity slow-wave structures of the type described above. The focusing means was then actually brought inside the vacuum envelope for the tube by extending the pole pieces of the aforementioned periodic permanent magnet focusing arrangement radially inwardly to the immediate vicinity of the electron stream and by hermetically sealing between each pair of adjacent pole pieces an annular nonmagnetic spacer element which is disposed radially within each magnet. The radially inwardly projecting portions of the pole pieces serve as the end walls of the slow-Wave circuit interaction cavities, While the inner circumferential surfaces of the annular spacer elements define the lateral walls of the interaction cavities, thereby producing a uniquely combined slow-wave structure and magnetic focusing arrangement.

The design of coupled cavity traveling-wave tubes for operation at higher frequencies has necessitated a corresponding reduction in the length of the interaction cavities. When a periodic permanent magnet focusing arrangement integrated with the slow-wave circuit interaction cavities in the manner described above is to be used in such high frequency tubes, the lengths of the magnets must be reduced by the same amount as the interaction cavities. If this reduction in magnet length becomes too great, however, demagnetizing problems may occur. In an attempt to avoid such demagnetization, periodic permanent magnet focusing schemes were devised in which the length of a single magnet extends over :1 interaction cavities, where n is a positive integer not less than two, so that the magnetic period becomes 2n times the electric period. However, if the magnetic period is increased sufiiciently, while maintaining a constant magnetic flux density and electron beam voltage, a stop band (i.e., no transmission of the electron beam) will be introduced.

Accordingly, it is an object of the present invention to provide a periodic permanent magnet electron beam focusing arrangement integrated with a traveling-wave tube coupled cavity slow-wave structure in which the magnet length is not limited by the electric period and, at the same time, in which there is no tendency toward the introduction of stop bands in beam transmission.

It is a further object of the present invention to provide a periodic permanent magnet electron beam focusing arrangement which, in addition to possessing the advantages set forth above, provides a greater axial magnetic focusing field for the same sized slow-wave structure than prior art focusing devices having the same magnet diameter.

It is a still further object of the present invention to provide a periodic permanent magnet electron beam focusing arrangement which is readily integratable with a coupled cavity slow-wave interaction circuit designed to operate at higher frequencies than has heretofore been possible, without the creation of demagnetization or beam transmission stop band problems.

In accordance with the foregoing objects, the focusing device of the present invention includes a plurality of axially aligned essentially annular permanent magnets magnetized axially and arranged with like poles of adjacent magnets confronting one another, with a plurality of disc-like ferromagnetic pole pieces interposed between and abutting adjacent magnets. The pole pieces project radially inwardly of the magnets and define aligned apertures in their central regions and coupling holes in regions radially outwardly of their central regions. Cavity defining means are disposed radially inwardly of each of the magnets for providing in the space defined by the inner circumferential surface of each magnet 11 interaction cavities, where n is a positive integer not less than two, of substantially equal axial extent successively disposed along the axis of the arrangement. The cavity defining means includes an annular portion of an axial extent equal to that of the magnet and of an outer diameter essentially equal to the inner diameter of the magnet, and further includes (n1) plate-like portions which project radially inwardly of the annular portion in planes substantially perpendicular to the axis at spacings of 1/ n of the pole piece spacing. Each plate-like portion defines an aperture in its central region which is aligned with the aligned apertures in the pole pieces to provide a passage for the electron stream, and further defines a coupling hole in a region readily outwardly of its central region for interconnecting adjacent interaction cavities so that an electromagnetic wave may propagate through the coupling holes in the plate-like portions and in the pole pieces. Each plate-like portion and each pole piece defines adjacent its central aperture a tubular portion which extends along the electron stream path. At least one tubular portion of each of the cavity defining means is made of ferromagnetic material, while the remainder of the cavity defining means is constructed of electrically conductive nonmagnetic material.

This arrangement produces, along the axis of the tube, a periodically varying magnetic field of a magnetic period equal to 211 times the electric period, with each ferromagnetic tubular portion of a plate-like portion of the cavity defining means causing an additional variation in axial magnetic field every magnetic half period. Since the magnetic period may be made several times greater than the electric period, the cavity length does not impose a lower limit on the magnet thickness, thereby allowing short cavities to be designed for high frequency operation. Moreover, the additional variations in magnetic field which occur every magnetic half period introduce a harmonic magnetic period which is sufficiently short to preclude the introduction of beam transmission stop bands which might otherwise occur on account of the relatively long fundamental magnetic period.

The foregoing, as well as other objects, advantages and characteristic features of the present invention, will be come readily apparent from the following detailed description of preferred embodiments of the invention when taken in conjunction with the accompanying drawings in which:

FIG. 1 is an overall view partly in longitudinal section and partly broken away of a traveling-Wave tube incorporating a magnetic focusing arrangement and slow-wave structure in accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line 22 of FIG, 1;

FIG. 3 is a longitudinal sectional view of one magnetic period of the magnetic focusing arrangement and slowwave structure of traveling-wave tube of FIG. 1, as taken along line 3-3 of FIG. 2;

FIG. 4 is a graph illustrating the axial magnetic fiux density as a function of axial distance along the tube for the arrangement shown in FIG. 3;

FIG. 5 is a longitudinal sectional view similar to FIG. 3 of a magnetic focusing arrangement and slow-Wave structure according to another embodiment of the present invention;

FIG. 6 is a graph illustrating the axial magnetic fiux density as a function of axial distance along the tube for the embodiment shown in FIG. 5;

FIG. 7 is a longitudinal sectional view similar to FIG. 3 of a magnetic focusing arrangement and slow-wave structure according to still another embodiment of the present invention;

FIG. 8 is a graph illustrating the axial magnetic flux density vs. axial distance along the tube for the embodiment of FIG. 7; and

FIGS. 9 and 10 are longitudinal sectional views similar to FIG. 3 of still further embodiments of the present invention.

Referring with more particularity to the drawings, and especially .to FIG. 1, the reference numeral 10 designates generally a traveling-wave tube which includes an arrangement 12 of magnets, pole pieces and spacer elements which function as a combined slow-wave structure for propagating an electromagnetic wave with a phase velocity substantially less than the velocity of light and a periodic focusing device focusing for the electron beam traversing the length of the slow-wave structure. Coupled to the input end of the arrangement 12 is an input wave guide transducer 14 which includes an impedance step transformer 16. A flange 18 is provided for coupling the assembled traveling-wave tube 10 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 18 may include a microwave window (not shown) transparent to microwave energy but capable of maintaining a vacuum within the traveling-Wave tube 10. At the output end of the arrangement 12 an output transducer 20 is provided which is substantially similar to the input transducer 14 and which includes an impedance step transformer 22 and a coupling flange 24, which elements are similar to the elements 16 and 18, respectively, of the input transducer 14. For vacuum pumping or out-gassing the traveling-wave 10 during manufacture, a double-ended pumping tube 26 is connected to both of the input and output waveguide transducers 14 and 26.

An electron gun 28 is disposed at one end of the traveling wave tube 10 which, although illustrated as the input end in FIG. 1, may alternatively be the output end if a backward wave device is desired. The electron gun 28 functions to project a stream of electrons along the axis of the tube 10 and may be of any conventional construction well known in the art. For details as to the construction of the gun 28 reference is made to Patent No. 2,985,792, entitled, Periodically Focused Traveling-Wave Tube, issued May 23, 1961, to D. J. Bates et al. and assigned to the assignee of the present invention and to Patent No. 2,936,393, entitled, Low Noise Traveling-Wave Tube, issued May 10, 1960 to M. R. Currie et al. and assigned to the assignee .of the present invention.

At the output end of the traveling-wave tube 10 there is provided a cooled collector structure 30 for collecting the electrons in the stream. The collector is conventional and may be of any form well known in the art. For details as to the construction of the collector, reference is made to the aforesaid Patent No. 2,985,792 and to Patent No.

2,860,277, entitled, Traveling-Wave Tube Collector Electrode, issued Nov. 11, 1958, to A. H. Iversen and assigned to the assignee of the present invention.

The construction of the combined slow-wave structure and focusing system for the traveling-wave tube are illustrated in more detail in FIGS. 2 and 3. A plurality of essentially annular permanent magnets 32 are interposed between a plurality of pole pieces 34 of a ferromagnetic material, which is preferably a vacuum melted high purity iron. As is illustrated in FIG. 3, the magnets 32 are arranged along the axis of the tube with like poles of adjacent magnets confronting one another so that a magnetic field reversal occurs at each pole piece 34. Moreover, for convenience during assembling of the tube, the magnets 32 may be diametrically split into two sections 32a and 32]) as shown in FIG. 2. The ferromagnetic pole pieces 34 extend radially inwardly of the magnets 32 to approximately the perimeter of the region adapted to contain the electron stream flowing along the axis 35 of the arrangement 12.

Disposed radially within each of the magnets 32 is a slow-wave circuit interaction cavity defining element designated generally by the numeral 36. Each element 36 includes an annular portion 38 of an axial extent equal to that of the magnet 32 within which it is located and an outer diameter essentially equal to the inner diameter of the magnet 32. Each element 36 also includes a plate-like portion 40 which projects radially inwardly from the annular portion 38 in a plane perpendicular to the axis 35 and located midway along the axial length of the annular portion 38. The cavity defining elements 36 and the portions of the pole pieces 34 extending radially inwardly of the annular portions 38 of the elements 36 define a series of interaction cells, or cavities, 42 which are successively disposed along the axis 35. The inner diameter of the annular portion 38 of the element 36 determines the radial extent of the interaction cell 42, while the axial length of the cavity 42 is determined by the distance between the pole piece 34 and the plate-like portion 40 of the element 35 defining respective ends of the cavity 42.

For interconnecting adjacent interaction cavities 42 an off-center coupling hole 44 is provided through each pole piece 34 and each plate-like portion 40 of the elements 36 to permit the transfer of traveling-wave energy from cavity to cavity. As is illustrated, the coupling holes 44 may be substantially kidney-shaped and may be alternately disposed 180 apart with respect to the axis 35. It should be pointed out, however, that the coupling holes 44 may be of other shapes and may be staggered in various other arrangements such as those disclosed in Patent No. 3,010,- 047, entitled, Traveling-Wave Tube, issued Nov. 21, 1961, to D. I. Bates and assigned to the assignee of the present invention. Thus, the elements 36 and the radially inner surfaces of the pole pieces 34 form a slow-wave structure for propagating traveling-wave energy in a serpentine path along the axially traveling electron stream so as to support energy exchange between the electrons of the stream and the traveling waves.

Each pole piece 34 and each plate-like portion 49 is constructed in such a manner that a short drift tube, or ferrule, 46 is provided at its inner radial extremity. The drift tube 46 is in the form of a cylindrical extension, or lip, protruding axially along the path of the electron stream from both broad surfaces of the pole piece 34 or the plate-like portion 40 on which it is formed. The drift tubes 46 are provided with central and axially aligned apertures 48 to provide a passage for the flow of the electron stream. Adjacent ones of the drift tubes 48 are separated by a gap 50 in which energy exchange between the electron stream and the wave energy traversing the slow-wave structure occurs. The annular portion 33 and the plate-like portion 40, but not the drift tube 46 adjacent the central aperture in the portion 40, of each cavity defining element 36 is constructed of a nonmagnetic electrically conductive material such as copper. However, the drift tube 46 adjacent the central aperture of the platelike portion 49 is made of a ferromagnetic material such as high purity iron.

The arrangement of FIG. 3 produces a magnetic flux density B along the axis 35 which varies as a function of axial distance x in the manner illustrated by the curve of FIG. 4. It may be seen that a periodically varying magnetic field distribution is produced along the axis of the tube, with zero magnetic flux density B occurring at the center of each pole piece 34 and with a fundamental magnetic period L equal to twice the distance between the centers of successive pole pieces 34. The ferromagnetic drift tubes 46 at the radially innermost regions of the plate-like portions 40 attract the magnetic lines of flux away from the axis 35 to provide regions of minimum magnetic flux density along the axis 35 at points where the planes of the plate-like portions 40 intersect the axis 35. These minimum flux regions, which are illustrated at 62 on the magnetic flux density curve 60 of FIG. 4, introduce a harmonic magnetic period of length 1 equal to the distance between the centers of successive ferromagnetic drift tubes 46. It may be seen that in the embodiment of FIG. 3 the harmonic magnetic period I is equal to the electric period P defined as the distance between the centers of the end walls 34 and 40 defining an interaction cavity 42, making the magnetic period L four times as great as the electric period P. Thus, the electric period may be made short for high frequency operation, while longer magnets 32 may be employed so that demagnetization does not occur. Moreover, the introduction of a harmonic magnetic period I substantially shorter than the fundamental magnetic period L precludes the introduction of beam transmission stop bands which might otherwise occur on account of the relatively long fundamental magnetic period L.

Although in the embodiment of FIG. 3 each cavity defining element 36 includes only a single plate-like portion 40 which divides the space within the inner circumferential surface of the associated magnet 32 into two interaction cavities 42, it is pointed out that in accordance with the principles of the present invention the element 36 may include (n-1) plate-like portions, where n is any positive integer not less than two, dividing the space within the inner circumferential surface of the magnet 32 into n interaction cavities 42 of substantially equal axial extent.

Thus, in the embodiment illustrated in FIG. 5, n is made equal to three by constructing each cavity defining element 36 with two plate-like portions 40 which project radially inwardly of the annular portion 38 in respective planes perpendicular to the axis 35 which are successively disposed along the axis 35 at spacings equal to one third of the pole piece spacing. As is the case for the embodiment of FIG. 3, the drift tubes 46 of the plate-like portions 40 of the elements 36 are made of ferromagnetic material, with the remaining portions of the elements 36 being of a nonmagnetic electrically conductive material. As is il lustrated by the curve 69' of FIG. 6, the embodiment of FIG. 5 provides two regions 62' of minimum magnetic flux density B between each pair of pole pieces 34, and the fundamental magnetic period L becomes six times as great as the harmonic magnetic period 1. Again, the harmonic magnetic period l is equal to the electric period P.

It is not necessary that all of the drift tubes 46 be of ferromagnetic material, and an embodiment in which alternate ones of the drift tubes are of nonmagnetic material is illustrated in FIG. 7. In this embodiment n is made equal to tour so that each element 36 defines three plateliice portions 40 which project radially inwardly of the annular portion 38 in respective planes perpendicular to the axis 35 which are successively located along the axis 35 at spacings of one-fourth of the distance between the centers adjacent pole pieces 34. The drift tube 46 of the middle plate-like portion 40 of each element 36 is made of ferromagnetic material, while the drift tubes 46' of the axially outer plate-like portions 40 of the element 36 are constructed of a nonmagnetic electrically conductive material, as is the remainder of the element 36. Thus, as is shown by the curve 60" of FIG. 8, for the embodiment of FIG. 7 a region 62 of minimum magnetic flux density B occurs at the center of each ferromagnetic drift tube 46 of a plate-like portion 40 but not at the nonmagnetic drift tubes 46'. Hence, while the fundamental magnetic period L of this embodiment is equal to eight times the electric period P, it is only four times the harmonic magnetic period I.

The embodiment illustrated in FIG. 9 is similar to that of FIG. 3 except that in the embodiment of FIG. 9 ferromagnetic material is used not only for the drift tube 46 of each element 36, but for the entire plate-like portion 40' as well, with only the annular portion 38 being of nonmagnetic material.

In the embodiment shown in FIG. 10 the ferromagnetic plate-like portions 40' extend radially outwardly to the inner circumferential surface of the magnets 32, dividing the outer regions of each cavity defining device 36 into a pair of nonmagnetic annular portions 38'.

Thus, it will be apparent that while the present invention has been shown and described with reference to particular embodiments, various changes and modifications obvious to those skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A device for focusing a stream of electrons along a predetermined path and for promoting interaction between said stream of electrons and an electromagnetic wave comprising: a plurality of axially aligned essentially annular permanent magnets magnetized axially and arranged with like poles of adjacent magnets confronting one another, a plurality of disc-like ferromagnetic pole pieces interposed between and abutting adjacent magnets, said pole pieces projecting radially inwardly of said magnets and defining aligned apertures in their central regions, each said pole piece further defining a coupling hole in a region radially outwardly of its central region, means disposed radially inwardly of each of said magnets for providing in the space defined by the inner circumferential surface of said magnet a plurality of interaction cavities of substantially equal axial extent successively disposed along said predetermined path, said means having an annular portion of an outer diameter essentially equal to the inner diameter of said magnet and at least one plate-like portion projecting radially inwardly of said annular portion in a plane substantially perpendicular to said predetermined path, each said platelike portion defining an aperture in its central region which is aligned with said aligned apertures of said pole pieces to provide a passage for said stream of electrons, each said plate-like portion further defining a coupling hole in a region radially outwardly of its central region for interconnecting adjacent interaction cavities whereby said electromagnetic wave may propagate through said coupling holes in said plate-like portions and in said pole pieces, each of said plate-like portions and each of said pole pieces defining adjacent its central aperture a tubular portion which extends along said predetermined path, and at least said tubular portion of each said means being of ferromagnetic material with the remainder of said means being of electrically conductive nonmagnetic material.

2. A device for focusing a stream of electrons along a predetermined path and for promoting interaction between said stream of electrons and an electromagnetic wave comprising: a plurality of axially aligned essentially annular permanent magnets magnetized axially and arranged with like poles of adjacent magnets confronting one another, a plurality of disc-like ferromagnetic pole pieces interposed between and abutting adjacent magnets, said pole pieces projecting radially inwardly of said magnets and defining aligned apertures in their central regions, each said pole piece further defining a coupling hole in a region radially outwardly of its central region, means disposed radially inwardly of each of said magnets for providing in the space defined by the inner circumferential surface of said magnet n interaction cavities, where n is a positive integer not less than two, of substantially equal axial extent successively disposed along said predetermined path, said means having an annular portion of an axial extent equal to that of said magnet and an outer diameter essentially equal to the inner diameter of said magnet, said means further having (n-l) plate-like portions projecting radially inwardly of said annular portion in planes substantially perpendicular to said predetermined path at spacings of 1/11 of the pole piece spacing, each said plate-like portion defining an aperture in its central region which is aligned with said aligned apertures of said pole pieces to provide a passage for said stream of electrons, each said plate-like portion further defining a coupling hole in a region radially outwardly of its central region for interconnecting adjacent interaction cavities whereby said electromagnetic wave may propagate through said coupling holes in said plate-like portions and in said pole pieces, each of said plate-like portions and each of said pole pieces defining adjacent its central aperture a tubular portion which extends along said predetermined path, and at least one tubular portion of each said means being of ferromagnetic material with the remainder of said means being of electrically conductive nonmagnetic material.

3. A device according to claim 1 wherein said tubular portion only of each said means is of ferromagnetic material with the remainder of said means being of electrically conductive nonmagnetic material.

4. A device according to claim 2 wherein n is a positive integer not less than three and wherein at least each said tubular portion of each said means is of ferromagnetic material with the remainder of said means being of electrically conductive nonmagnetic material.

5. A device according to claim 2 wherein n is a positive integer not less than three and wherein at least one of said tubular portions of each said means is of ferromagnetic material with the remainder of said means including at least one other of its tubular portions being of electrically conductive nonmagnetic material.

6. A device for focusing a stream of electrons along a predetermined path and for promoting interaction between said stream of electrons and an electromagnetic wave comprising: a plurality of axially aligned essentially annular permanent magnets magnetized axially and arranged with like poles of adjacent magnets confronting one another, a plurality of disc-like ferromagnetic pole pieces interposed between and abutting adjacent magnets, said pole pieces projecting radially inwardly of said magnets and defining aligned apertures in their central regions, each said pole piece further defining a coupling hole in a region radially outwardly of its central region, means disposed radially inwardly of each of said magnets for providing in the space defined by the inner circumferential surface of said magnet a plurality of interaction cavities of substantially equal axial extent successively disposed along said predetermined path, said means including an annular member of electrically conductive nonmagnetic material of an axial extent equal to that of said magnet and of an outer diameter essentially equal to the inner diameter of said magnet and at least one platelikc member of ferromagnetic material extending radially inwardly from the inner circumferential surface of said annular member to the vicinity of said predetermined path in a plane substantially perpendicular to said predetermined path, each said plate-like member defining an aperture in its central region which is aligned with said aligned apertures of said pole pieces to provide a passage for said stream of electrons, each said plate-like member further defining a coupling hole in a region radially outwardly of its central region'for interconnecting adjacent interaction cavities whereby said electromag netic wave may propagate through said coupling holes in said plate-like members and in said pole pieces, and each of said plate-like members and each of said pole pieces defining adjacent its central aperture a tubular portion which extends along said predetermined path.

7. A device for focusing a stream of electrons along a predetermined path and for promoting interaction between said stream of electrons and an electromagnetic wave comprising: a plurality of axially aligned essentially annular permanent magnets magnetized axially and arranged with like poles of adjacent magnets confronting one another, a plurality of disc-like ferromagnetic pole pieces interposed between and abutting adjacent magnets, said pole pieces projecting radially inwardly of said magnets and defining aligned apertures in their central regions, each said pole piece further defining a coupling hole in a region radially outwardly of its central region, means disposed radially inwardly of each of said magnets for providing in the space defined by the inner circumferential surface of said magnet a plurality of interaction cavities of substantially equal axial extent successively disposed along said predetermined path, said means including a plurality of annular members of electrically conductive nonmagnetic material of an outer diameter essentially equal to the inner diameter of said magnet and a plate-like member of ferromagnetic material interposed between and abutting each pair of adjacent annular mem- 1 stream of electrons, each said plate-like member further defining a coupling hole in a region radially outwardly of its central region for interconnecting adjacent interaction cavities whereby said electromagnetic wave may propagate through said coupling holes in said plate-like members and in said pole pieces, and each of said plate-like members and each of said pole pieces defining adjacent its central aperture a tubular portion which extends along said predetermined path.

References Cited UNITED STATES PATENTS 1/1959 Pierce 313-84 X 9/1966 Verger 313-84 X HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner. 

1. A DEVICE FOR FOCUSING A STREAM OF ELECTRONS ALONG A PREDETERMINED PATH AND FOR PROMOTING INTERACTION BETWEEN SAID STREAM OF ELECTRONS AND AN ELECTROMAGNETIC WAVE COMPRISING: A PLURALITY OF AXIALLY ALIGNED ESSENTIALLY ANNULAR PERMANENT MAGNETS MAGNETIZED AXIALLY AND ARRANGED WITH LIKE POLES OF ADJACENT MAGNETS CONFRONTING ONE ANOTHER, A PLURALITY OF DISC-LIKE FERROGMAGNETIC POLE PIECES INTERPOSED BETWEEN AND ABUTTING ADJACENT MAGNETS, SAID POLE PIECES PROJECTING RADIALLY INWARDLY OF SAID MAGNETS AND DEFINING ALIGNED APERTURES IN THEIR CENTRAL REGIONS, EACH SAID POLE PIECE FURTHER DEFINING A COUPLING HOLE IN A REGION RADIALLY OUTWARDLY OF ITS CENTRAIL REGION, MEANS DISPOSED RADIALLY INWARDLY OF EACH OF SAID MAGNETS FOR PROVIDING IN THE SPACE DEFINED BY THE INNER CIRCUMFERENTIAL SURFACE OF SAID MAGNET A PLURALITY OF INTERACTION CAVITIES OF SUBSTANTIALLY EQUAL AXIAL EXTENT SUCCESSIVELY DISPOSED ALONG SAID PREDETERMINED PATH, SAID MEANS HAVING AN ANNULAR PORTION OF AN OUTER DIAMETER ESSENTIALLY EQUAL TO THE INNER DIAMETER OF SAID MAGNET AND AT LEAST ONE PLATE-LIKE PORTION PROJECTING RADIALLY INWARDLY OF SAID ANNULAR PORTION IN A PLANE SUBSTANTIALLY PERPENDICULAR TO SAID PREDETERMINED PATH, EACH SAID PLATELIKE PORTION DEFINING AN APERTURE IN ITS CENTRAL REGION WHICH IS ALIGNED WITH SAID ALIGNED APERTURES OF SAID POLE PIECES TO PROVIDE A PASSAGE FOR SAID STREAM OF ELECTRODES EACH SAID PLATE-LIKE PORTION FURTHER DEFINING A COUPLING HOLE IN A REGION RADIALLY OUTWARDLY OF ITS CENTRAL REGION FOR INTERCONNECTING ADJACENT INTERACTION CAVITIES WHEREBY SAID ELECTROMAGNETIC WAVE MAY PROPAGATE THROUGH SAID COUPLING HOLES IN SAID PLATE-LIKE PORTIONS AND IN SAID POLE PIECES, EACH OF SAID PLATE-LIKE PORTIONS AND EACH OF SAID POLE PIECES DEFINING ADJACENT ITS CENTRAL APERTURE A TUBULAR PORTION WHICH EXTENDS ALONG SAID PREDETERMINED PATH, AND AT LEAST SAID TUBULAR PORTION OF EACH SAID MEANS BEING OF FERROMAGNETIC MATERIAL WITH THE REMAINDER OF SAID MEANS BEING OF ELECTRICALLY CONDUCTIVE NONMAGNETIC MATERIAL. 